Method for producing pyrophoric metal alloy powders



United States Patent 3,539,334 METHOD FOR PRODUCING PYROPHORIC METAL ALLOY POWDERS Walter V. Goeddel, Poway, and Robert J. Akins, San

Diego, Calif., assignors to Gulf General Atomic Incorporated, San Diego, Calif., a corporation of Delaware No Drawing. Filed Aug. 13, 1968, Ser. No. 752,172 Int. Cl. B22f 9/00; C22b 59/00 US. Cl. 75.5 6 Claims ABSTRACT OF THE DISCLOSURE Friable pyrophoric metal alloys are produced by combining a pyrophoric metal or metal alloy with an alloying material. The friable pyrophoric metal alloys are thereafter comminuted to provide pyrophoric metal alloy powders. A preferred pyrophoric metal alloy is produced by combining misch metal with an alloying material selected from zinc, silicon, carbon and combinations thereof. Other alloying materials such as tungsten, cadmium, lead or combinations thereof may also be used in the pyrophoric metal alloy to provide control of density.

The present invention relates generally to pyrophoric powders and more particularly relates to a method for making pyrophoric powders.

Pyrophoric materials are well known and have the property of emitting sparks when scratched or struck with a hard object such as steel. Pyrophoric metals are commonly used in cigarette lighter flints and in lighters for gas welding apparatus.

It is proposed to provide pyrophoric materials in a finely divided, particulate form, referred to herein as a powder. Such pyrophoric powders are useful as incendiary materials in the manufacture of incendiary weapons.

Accordingly, it is an object of the present invention to provide a method for the manufacture of pyrophoric powders. It is another object of the present invention to provide a method for the preparation of slow-burning powders of pyrophoric materials. It is a further object of the invention to provide a method for preparing pyrophoric powders that are suitable for use in incendiary devices. It is a still further object of the present invention to provide a method for preparing misch metal powders which are suitable for use in incendiary devices.

These and other objects of the present invention will become more apparent from the following detailed disclosure and the appended claims.

Generally, pyrophoric metal powders are produced by combining a pyrophoric metal or metal alloy with an alloying material so as to provide a comminutable pyrophoric metal alloy. It has been found that certain misch metal powders have the slow-burning characteristics desirable in incendiary weapons. Misch metal is a well known metal alloy that consists of a crude mixture of cerium, lanthanum, neodymium, praseodymium and other rare earth metals in the approximate ratio in which they occur in monazite sand or bastnasite ore and is usually obtained by electrolysis of the fused mixed chlorides or oxides of the rare earth metals. Misch metal is too ductile to be ground unless sophisticated and costly techniques are used; however, by alloying misch metal with a suitable material, a friable product is provided. Thereafter, the friable alloy is comminuted in the absence of oxygen or water vapor to provide the pyrophoric metal alloy powder. The alloying materials for providing a friable pyrophoric metal alloy in combination with misch metal are preferably selected from silicon, carbon, zinc, and combinations thereof. Other materials, such as tungsten, cadmuim,

lead, or combinations thereof, may be used in the pyrophoric metal alloy to provide better control of density.

It is desirable in producing materials which are to be used for ballistic ordnance purposes, such as artillery shells or bombs, to be able to closely control the density of the materials so as to provide ordance devices with charges which are uniform and which have weights that are compatible with other materials. By the process of the present invention, the density of the pyrophoric metal alloy powders produced may be varied to provide a desired density within a range of from about 5.0 grams/cc. to about 7.5 grams cc. Such variation in density is achieved by the selection of the alloying material and/ or by varying the levels at which the alloying material is added to the pyrophoric metal.

A friable pyrophoric metal alloy is prepared by providing a melt of the pyrophoric metal and the alloying material in a suitable crucible and thereafter cooling the melt. The melt may be made by heating the pyrophoric metal and the alloying material together to a suitable temperature, by first melting the pyrophoric material and then adding the alloying material, or by first melting the alloying material and then adding the pyrophoric material. Because elevated temepratures are necessary, an inert atmosphere is provided during the heating to prevent combustion of the pyrophoric metal. The inert atmosphere may 'be provided by establishing a vacuum of less than about 0.05 mm. absolute pressure, but it is preferably provided by surrounding the crucible containing the pyrophoric metal and the alloying material with an inert gas.

To achieve the desirable friable characteristics in the pyrophoric alloy, the alloying material is added at a sufficient level for the particular alloying material. For example, when silicon is used as the alloying material, it is necessary to add the silicon to the pyrophoric metal alloy in the melt at a level of at least about 15 percent by weight of the pyrophoric metal, whereas when zinc is used as the alloying material a suitable friable alloy melt may be produced only when the zinc is added at levels of at least about 25 percent by weight.

The preferred pyrophoric metal for making such powders is misch metal. As previously indicated, misch metal is a combination of rare earth metals at the levels of the metals usually found in monazite sand or bastnasite ore. Misch metal, as it is produced from the basic ores, is considered unsuitable for use in an incendiary device because misch metal is too soft to permit practical grinding and oxidizes too readily. It is considered a feature -of the present invention that not only is a friable material produced by addition of the alloying material which is capable of being easily comminuted or ground to a desired fine particle size, but also that the resultant alloy has the desired relatively slow-burning characteristics. Preferably, the burning characteristics of the resultant powder should be such that substantially all (at least about 95 percent) of the powder will oxidize within between about 45 and about minutes after it is ignited in air at ambient temperatures.

The ability both to vary the density of the pyrophoric alloy and to produce powders within desired particle size ranges is also an important feature of the present invention. By producing a pyrophoric alloy powder having a desired density and also having a particle size within a desired range, the bulk density of the pyrophoric alloy powder may be varied within desired limits so as to provide a powder that is well suited for ordnance purposes.

The friability of the pyrophoric alloys of the invention are generally considered suitable if the pyrophoric alloy material is readily comminuted to the desired particle size by the use of conventional size reduction equipment,

such as ball mills or hammer mills, and by conventional techniques. Friability meeting this specification may be obtained in a misch metal alloy by incorporating silicon at a level of at least about 15 percent by weight, by adding carbon at a level of at least about 8 percent by weight, by adding zinc at a level of at least about 25 percent by weight, or by adding various combinations thereof. All percentages being by weight of the misch metal, unless otherwise indicated. Zinc when used to vary the density and produce a friable pyrophoric alloy is used at levels of up to about 40 percent by weight, carbon up to about 10 percent by weight and silicon up to about 20 percent by weight. It has further been found that various other materials may be included with the silicon, carbon or zinc to provide a better density control. In general, any metallic material having a density in the range of about 5.0 grams/ cc. to about 19.5 grams/ cc. may be used to provide density control. Preferred materials are cadmium, tungsten and lead or combinations thereof. However, it should be understood that the desired characteristics of the pyrophoric alloy do not result from the addition of the density controlling materials, such as cadmium, lead or tungsten or compounds thereof, but only in combination with the zinc, silicon or alloying material. Cadmium, lead or tungsten or compounds thereof, when used to vary the density of the pyrophoric alloy, are used at levels which result in an alloy having a density of between about 6.5 and 7.5 gm./cm.

In producing the pyrophoric alloy, the misch metal may be introduced in small pieces to a suitable crucible along with the alloying material. The crucible is heated in an inert atmosphere to a temperature whereat both the misch metal and the alloying material are molten. After cooling the melt, the friable pyrophoric alloy mass is then ground to a powder using conventional grinding methods and operating under an inert atmosphere. Desired particle sizes or range of sizes are obtained by any suitable procedure, such as screening.

The following examples further illustrate various features of the present invention, but should be understood to in no way limit the scope of the invention, which is defined in the appended claims.

EXAMPLE I A misch metal-silicon pyrophoric alloy powder is produced by the following process:

480 grams of misch metal pellets are placed in a tantalum crucible along with 120 grams of elemental silicon. The crucible is disposed in a vacuum induction furnace, and the furnace is evacuated while cold to a pressure of 25 microns of the Hg. Power is applied and maintained until the crucible attains a red heat. The absolute pressure increases due to vaporization, and the power setting is maintained constant until the pressure stabilizes. The furnace is then filled with helium to establish a pressure of 1 to 2 p.s.i.g. The power is then increased until a temperature of about ISO-0 C. is reached, as measured by an optical pyrometer, the temperature being increased at a rate of about 300 C. per minute. This temperature is held for five minutes and then reduced to 1350 C., where it is held for one to two minutes. The temperature is then again raised to 00 C. for one minute. The power is then shut off, and the crucible is allowed to cool in the furnace.

The misch metal-silicon alloy is removed from the crucible and is found to be a frangible material. The alloy is ground in an ore crusher to a particle size of less than 40 microns. The resultant powder has an absolute density of 5.47 grams per cc. and ignition temperature of the powder is about 130 C. The powder burns at a temperature of 900 C., and after about 60 minutes the powder is almost completely oxidized. The burning characteristics are considerably slower than Thermit mixtures, and no sparks are emitted.

EXAMPLE II A misch metal-zinc pyrophoric alloy powder is produced by the following process:

581 grams of zinc are placed in a tantalum crucible which is disposed in a resistance heated furnace. The furnace is provided with an inert atmosphere, e.g., helium or argon. The temperature of the furnace is raised so as to melt and raise the temperature of the zinc to about 700 C. 1235 grams of misch metal are then added to the molten zinc through a double valve arrangement, which does not result in polution of the inert atmosphere.

The crucible containing the misch metal-zinc alloy is allowed to cool to room temperature, whereupon it is removed from the protective atmosphere and the ingot removed. The ingot is friable and is ground to a particle size of about 40 microns. The absolute density is found to be 6.65 g./cm.

The powder ignites easily in air and burns with a bril liant display of sparks. After the sparks cease the hard cake of fused powders continues to oxide slowly. After 4560 minutes the powder is essentially consumed.

EXAMPLE III The procedure of Example I repeated to produce a misch metal-carbon alloy containing 48 grams of carbon and 552 grams of misch metal. The production of the melt and the grinding of the powder are performed substantially as set forth in Example I. The ignition temperature of the powder is about 30 C. The powder burns at about 900 C., and after about 60 minutes, the powder is substantially completely oxidized. The powder has satisfactory characteristics for use in incendiary device.

Although the invention has been described generally with regard to the use of misch metal as the principal metal to be alloyed, uranium has similar pyrophoric characteristics and may be substituted for misch metal in producing satisfactory pyrophoric powders. Although it is contemplated that misch metal will be employed in the composition in which it is obtained by electrolysis of either rare earth chlorides or oxides, for purposes of a somewhat more precise definition, misch metal may be considered to include other alloys or primarily rare earth metals, regardless of their origin, which contain cerium in the largest amount, neodymium lanthanum, the next largest amounts and also contain significant amounts of praseodyrnium and samarium.

Various of the features of the invention are set forth in the following claims.

What is claimed is:

1. A method for making a pyrophoric powder which method comprises melting together, in an atmosphere substantially free of oxygen, misch metal and an alloying material selected from the group consisting of zinc, silicon, carbon and combinations thereof, so as to provide a melt, said alloying material being present at a level sufficient to provide an alloy which is friable when cooled, cooling said melt to provide a solid alloy and comminuting said solid alloy in the absence of oxygen to provide a pyrophoric alloy powder.

2. A method in accordance with claim 1 wherein said comminuting is carried out to provide particles not greater than about microns in size.

3. A method in accordance with claim 1 wherein the alloying material is silicon which is present in an amount between about 15 and about 20 weight percent based upon the Weight of said misch metal.

4. A method in accordance with claim 1 wherein said alloying material is carbon which is present in an amount between about 8 and about 10 weight percent based upon the weight of said misch metal.

5. A method in accordance with claim 1 wherein said alloying material is zinc which is present in an amount between about 25 and about 40 weight percent based upon the weight of said misch metal.

6. A method in accordance with claim 1 wherein an additional alloying material selected from the group con sisting of cadmium, tungsten or lead or combinations 5 6 thereof in elemental or oxygen free compound form is in- FOREIGN PATENTS eluded as a part of said melt, said alloying material being 15 507 6/1910 Great Britain present at a level suflicient to provide an alloy having a 16853 7/1903 Great Britain density of from between about 6.5 and about 7.5 gms. 1,044,783 7/1963 GreatBritain per cm.-".

References Cited 5 L. DEWAYNE RUTLEDGE, Primary Examiner UNITED STATES PATENTS W. W. STALLARD, Assistant Examiner 1,023,208 4/1912 Lesmiiller 75152 US. Cl. X.R.

Re. 25,558 4/1964 Bungardt 75152 10 75152 

